1. Field of the Invention
The invention relates generally to the field of measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole. More particularly, the invention presents method and apparatus for increasing signal-to-noise ratio and depth of investigation of the nuclear magnetic resonance (NMR) measurements.
2. Background Art
A NMR well logging instrument uses a static magnetic field imposed on an earth formation to align nuclear spin magnetic moments of protons or other nuclei present in the earth formations. The static magnetic field may be naturally generated as in case of using the earth's magnetic field or produced by a permanent magnet or an electromagnet. The aligned spin magnetic moment is brought into excited state either by applying an RF magnetic field or switching the static magnetic field. RF voltages are induced in the receive antenna as a result of precessional rotation of nuclear spin axes of hydrogen protons or other nuclei about the static magnetic field with characteristic resonance or Larmor frequency corresponding to the static magnetic field strength.
Typical NMR tool using precession of the polarized nuclear magnetic moments about the earth's magnetic field comprises a source of static magnetic field polarizing nuclei. The field is turned off to allow free or driven precession about the earth's magnetic field. U.S. Pat. No. 3,538,429 issued to Baker, and U.S. Pat. No. 4,035,718 issued to Chandler give examples of the technique that uses abrupt (non-adiabatic) turning off the static magnetic field and detecting signal generated by free precession of nuclear spin magnetic moments. An improved technique is described in U.S. Pat. No. 3,667,035 issued to Slichter. The polarizing field is terminated adiabatically within a predetermined time interval less than the characteristic thermal relaxation time of the selected nuclei. Prior to the termination an alternated magnetic field directed transversal to the earth's magnetic field at a frequency corresponding to the Larmor precession frequency is applied. As a result of application of the alternating magnetic field and adiabatic termination of the polarizing magnetic field the nuclear spins are sweeping onto condition of driven resonance in the earth's magnetic field. Then the alternating field is abruptly terminated to facilitate free precession of the nuclei in the earth's magnetic field. U.S. Pat. No. 6,366,086 issued to Pabitra addresses an issue of an undesirable signal acquired from borehole fluid. Drawbacks of the methods of this type are poorly defined excitation region as well as relatively low SNR due to power-consumption-limited intensity of switchable static magnetic field.
Another approach to the NMR measurement in the borehole is represented by and U.S. Pat. No. 4,717,878 issued to Taicher et al., U.S. Pat. No. 5,055,787 issued to Kleinberg et al., and U.S. Pat. No. 6,452,388 issued to Reiderman, et al. The approach employs using a permanent magnet that generates polarizing magnetic field that aligns nuclear spin magnetic moment. The angle between the nuclear magnetization is then changed by applying a pulsed radio-frequency (RF) magnetic field at a frequency corresponding to the static magnetic field magnitude at a predetermined distance from the NMR tool. A sequence of RF pulses can be designed to manipulate the nuclear magnetization in order to acquire relaxation property of the earth formation. For the NMR well logging the most common sequence is the CPMG sequence that comprises one excitation pulse and a plurality of refocusing pulses. Due to an extremely inhomogeneous magnetic field the NMR excitation is constrained to a thin shell resulting in relatively shallow region of investigation and sensitivity to any lateral displacement of the logging instrument.
Yet another approach to NMR logging measurement is disclosed in U.S. Pat. No. 6,166,543 issued to Sezginer et al. and U.S. Pat. No. 6,133,753 issued to Hurlimann et al. The approach is based on non-resonant excitation and refocusing and has an advantage of having a relatively large excitation volume compared to resonant excitation methods described above. The method utilizes capability of generating first magnetic field and second magnetic field that is substantially orthogonal to the first magnetic field. Nuclear magnetization is polarized by the first magnetic field. Fast switching between the first and the second magnetic field causes free precession of the nuclear spin magnetic moments about the second magnetic field. A change in polarity of the second magnetic field reverses the direction of precession thereby generating a train of gradient echoes. Practical consideration of power consumption related to fast switching between magnetic fields and maintaining first magnetic field during polarization phase place clear limitations on the strength of the first and the second magnetic fields. The limitations cause relatively low signal-to-noise ratio for measurements at a desired depth of investigation.
Known in the art are electrically or mechanically switchable magnets used in lifting magnetized objects. The magnets do not require a source of constant current. Consequently no energy losses are encountered to maintain magnetization of the magnet. A system representing switchable magnets is disclosed for example in U.S. Pat. No. 6,229,422 issued to Pignataro. The magnet assembly comprises two magnets with coil around one of them. The magnets are typically connected with a magnetically permeable frame. Energizing the coil in one direction reverses the polarization one magnet, thereby effectively short circuiting flux produced by the second magnet. This terminates holding the object. Energizing the coil in the opposite direction causes parallel polarization of the magnets thereby switching the magnet assembly into holding mode. The known in the art switchable magnet systems are not suitable for fast switching the magnetic field. They also unable to provide complete reversal of the magnetic field as well as complete zeroing of the magnetic field.
Thus known in the art instruments do not give a satisfactory solution for effective generating of strong, fast switchable static magnetic field that would facilitate deep and high SNR NMR measurement in downhole environment.